This invention relates in general to spread spectrum communication systems, and in particular to a method and apparatus for assigning an optimum link for data packet transmission in a supplemental channel (of a CDMA 2000 system) or a downlink shared channel (in a UMTS system) during soft handoff in a spread spectrum system.
Communication systems are well known and consist of many types including land mobile radio, cellular radiotelephone, personal communication systems, and other communication system types. Within a communication system, transmissions are conducted between a transmitting device and a receiving device over a communication resource, commonly referred to as a communication channel. Transmissions used to consist primarily of voice signals and low speed data signals. More recently, however, it has proposed to use radio communication systems for high-speed data signals. For ease of operation, it is preferable to have the data transmission capability overlay the existing voice communication capability, such that its operation is essentially transparent to the voice communication system while still utilizing the communication resources and other infrastructure of the voice communication system.
Two such communication systems currently being developed with transparent data transmission capabilities are spread spectrum communication systems known as the next generation Code-Division Multiple Access (CDMA) cellular communication system, or cdma2000, and the next generation Global System for Mobile Communications (GSM), or Universal Mobile Telephone System (UMTS). Within these well known spread spectrum communication systems, all subscriber unit transmissions occur simultaneously within a frequency band, and all base station transmissions occur simultaneously within a frequency band. Therefore, a received signal at a base station or a subscriber unit comprises a multiplicity of frequency and time overlapped coded signals form individual subscriber units or base station units, respectively. Each of these signals is transmitted simultaneously at the same radio frequency (RF) and is distinguishable only by its specific encoding (channel).
Within spread spectrum communication systems, a subscriber is typically assigned at least one link that is used to communicate information between the communication system and the subscriber unit. Each link comprises channels that are assigned to communicate information between the subscriber unit and a (geographic) sector of a base transmitter site. Every link includes one channel called a pilot signal that is used for several purposes, including setting up and monitoring the signal strength of the link. The link can also comprise what is named herein a xe2x80x9cfundamental channelxe2x80x9d that is dedicated only to one subscriber unit during the duration of a voice call and is used to transfer voice information between the subscriber unit and the communication system. The fundamental channel as named herein is called a fundamental channel in the cdma200 communication system but is called a dedicated channel in the UMTS communication system. The link can also comprise what is called herein a supplemental channel that is assigned to a subscriber unit to transfer high-speed digital information between a subscriber unit and the communication system, but the assignment lasts only as, long as needed to accomplish the transfer of the data. The supplemental channel as named herein is called a supplemental channel in the cdma200 communication system but is called a shared channel in the UMTA communication system. Although the supplemental channel of the cdma2000 and the shared channel of the UMTS system have some quite different characteristics, they also share some common characteristics. The same is true for the fundamental channel of the cdma2000 system and the dedicated channel of the UMTS system.
At some times during which a subscriber unit is being used in the communication system, only one link is assigned to the subscriber unit, because the strength of the link from the base transmitter site to the subscriber unit (the downlink) has been determined to be sufficient to provide consistent high quality service. However, at other times, the subscriber is located within the communication system at a point where no single downlink can provide consistent high quality service, but lower quality downlinks are possible to more than base transmitter site sectors. Both conditions can occur at different times during one voice call. A unique aspect of spread spectrum communication systems is that the use of the spread spectrum modulation and coding technique allows a combining of the multiple received signals that carry the same information. The combining adds together the signal strengths of the individual signals, and in many instances is capable of providing a high quality received signal from the several downlinks. This combining is typically used in spread spectrum communication systems for the fundamental channels (the voice channels) until one of the downlinks becomes strong enough to provide high quality service by itself, at which time all other links are dropped. The combining is, however, not allowed for the supplemental channels, because they occupy a substantial portion of the RF resources, in terms of power and bandwidth, and because they are typically very short compared to voice calls. The period of time during which the combining takes place is called the xe2x80x9csoft handoff periodxe2x80x9d or xe2x80x9chandoff periodxe2x80x9d and the operation is called the xe2x80x9chandoffxe2x80x9d by those of ordinary skill in the art, because it commonly occurs when a new single link is assigned and an old link is dropped. The terms xe2x80x9csoft handoff periodxe2x80x9d and xe2x80x9chandoff periodxe2x80x9d are also used herein for the time period during which a fundamental channel would operate in the combining mode but is not assigned to the subscriber unit (for example, because there is no voice call occurring). During a handoff period, the links that are assigned or would be assigned to a subscriber unit for combining are called the active links.
When a subscriber unit is not in operating during a handoff period, a supplemental channel is assigned to the same link as the pilot channel, and the quality of service for the supplemental channel is will be satisfactory. However, during a handoff, there is a problem in accurately determining a best downlink for transmission of a data packet using a supplemental channel because of a combination of factors: the shortness of typical data packets, an uncertainty as to when the data packet will be transmitted due to system queuing delays, and the difficulty of knowing the signal strength of the active links at them moment the data packet is transmitted.
Thus, what is needed is a technique for selecting a best link for supplemental channel assignment during a handoff period in a spread spectrum system.